1
Power Meter Front End Design:
The Delta Connection
Atmel’s AT73C500 + AT73C501-based
meter chipset measures power and
energy in three-phase systems or, alter-
natively, the chipset can be set to
operate in a mode where it measures
three separate, single-phase connec-
tions. In normal modes of operation, the
chipset measures Y-connected three-
phase, four-wire environments, but there
is no dedicated operating mode for
Delta-connected three-phase, three-wire
systems. This application note describes
how to implement the chipset in three-
phase, three-wire environments.
Three-Phase Basics
To understand the measurement
method, it is important to outline some of
the following basic concepts.
Terminology
Voltages (and currents) in a three-phase
system are referred to as either phase
voltages (currents) or main voltages
(currents), depending on the point of
reference.
•A Phase Voltage(1) (UP1, UP2 and UP3)
is always referenced to the neutral
wire. In systems where there is no
physical neutral wire, the phase
voltages are referenced to a virtual
neutral.
Note: 1. Also called neutral voltage. This
should not be confused with the
ground neutral that is at zero
potential.
The Main Voltage (U1, U2 and U3) is
defined as the voltage between two of
the phase conductors.
•A Phase Current (IP1, IP2 and IP3) is
the current flowing through the load
impedance. Depending on the type of
connection, this current is not
necessarily directly measurable from
the meter connection points, but must
be derived from the vector sum of
other currents.
The Main Current (I1, I2 and I3) is
defined as the current that flows
through the phase conductor.
Depending on the type of connection,
this current is not necessarily the
same that flows through the load.
The Star Connection
A typical three-phase, four-wire service
with a grounded neutral is illustrated in
Figure 1. The system shown is com-
monly referred to as a YN-configuration,
with the subscript denoting there is a
neutral wire present. The wiring method
is sometimes also referred to as Star
connection.
In Figure 1, the circuit on the left illus-
trates the power generator and the
circuit on the right the load. In this type of
connection, the energy meter would
interface to measure the phase volt-
ages (UP1, UP2 and UP3) and phase
currents (IP1, IP2 and IP3).
In a balanced (all loads are equal) Star
connection, the neutral line carries no
current and the main current is equal to
the phase current. The main voltage is
related to the phase voltage as follows:
P
U3U ×=
Rev. 1680A07/00
Power Meter
Front End
Design:
The Delta
Connection
Application
Note
Meter Front End Design
2
Figure 1. Three-phase, Four-wire, YN-connection
The Delta Connection
The number of wires may be reduced from four to three, if
the load is wired in Delta-configuration. It is noted that the
generator does not necessarily have to be wired in Delta-
configuration, it can also be Star-connected. An example of
both the generator and the load-connected in Delta-config-
uration is illustrated in Figure 2.
The connection in Figure 2 contains no neutral wire and the
inherent voltage marking therefore refers to main voltages
(measured between phase conductors). When recalculat-
ing main voltages to phase voltages, the voltages are
referenced to a virtual neutral wire.
It is noted, that in a balanced Delta connection, the relation-
ship between phase current and main current is:
Figure 2. Three-phase, Three-wire, Delta Connection
P1
U
L1
L2
L3
N
P2
U
P3
U
1
U
2
U
3
U
1
I
2
I
3
I
N
I
P1
I
P3
I
P2
I
1
Z
2
Z
3
Z
P
I3I ×=
VIRTUAL GROUND POTENTIAL
P1
U
L1
L2
L3
P2
U
P3
U
1
U
2
U
3
U
1
I
2
I
3
I
N
I
P1
I
P3
I
P2
I
31
Z
12
Z
23
Z
Meter Front End Design
3
Front End Connections
The schematic diagram on page 7 shows a method for con-
necting the A/D converter to three-wire service. The
example Delta connection is rated for a 230/400V system,
but may easily be adapted to any other rating.
The Voltage Front End
The two transformers (Tr1 and Tr2) together with the resis-
tor network R1 through R6 convert the main voltages to
phase voltages. The conversion factor (the number of pri-
mary windings per number of secondary windings) of the
transformers is 1:1.
Resistors R1, R2 and R3 must be scaled to match the volt-
age of the system in order for the ADC inputs not to exceed
the maximum peak rating of 1 volt. ADC voltage maximum
should be reached at maximum main voltage of the system
front end. It may be assumed that the main voltage is equal
for all three phases and, therefore, that the maximum input
voltage to the converter, ûIN, can be calculated from any
one of the following:
We assume a system where the phase voltage is 230V
and, consequently, the main voltage 400V. The analog
front end should allow for a 15% voltage overhead, as
specified in IEC standards (see Table 1).
To achieve the ratings in Table 1, we start by defining resis-
tors R4 = R5 = R6 = 1 kohm, and get:
Using a standard valued resistor of 390 kohm, the maxi-
mum voltage for this configuration will be approximately
275V (i.e., about 390V peak amplitude). The voltage maxi-
mum should later be fine-tuned to 270V during factory
calibration.
The above calculations may be repeated to adapt the
design for any voltage system, but the maximum voltage of
the meter front end must be considered when reading data
or pulses from the meter. It should be noted that the default
bit resolution of data registers and the meter constant of
pulse outputs are based on a 270V maximum voltage.
It should also be noted that the DSP now measures phase
voltages of a Delta-connected load. For example, the meter
software of the ATEK5003 (The Evaluation Kit for the
AT73C500 chipset) displays RMS voltage values with
respect to the phase voltage and not the main voltage..
2U
R6R3
R6
2U
R5R2
R5
2U
R4R1
R4
u
ˆPFSPFSPFSIN ××
+
=××
+
=××
+
=
380kohmR41
u
ˆ
2U
R3R2R1
IN
PFS ×
×
===
Table 1. Voltage Ratings at Various Circuit Nodes
Path Node Symbol Nominal Maximum Unit
Voltage
Phase1
Phase2
Phase3
U1
U2
U3
400 460 VRMS
Tr1 / 3 - G N D
Tr1 / 4 - G N D
Tr2 / 4 - G N D
UP1
UP2
UP3
230 270 VRMS
C1
C2
C3
VI1
VI2
VI3
0.602 0.707 VRMS
Current
Tr 3 / P r i m a r y
Tr 4 / P r i m a r y
Tr 5 / P r i m a r y
I1
I2
I3
-80A
RMS
C4
C5
C6
CI1
CI2
CI3
-0.707V
RMS
Meter Front End Design
4
The Current Front End
Current transformers Tr3, Tr4 and Tr5 are used for sensing
the phase current, which goes through the primary winding.
The transformers should have a conversion factor, M, such
that, at maximum rated phase current, the secondary cur-
rent, which goes through the shunt resistors (R7, R8 and
R9) does not produce more than 1-volt peak amplitude at
the ADC input. The higher the conversion factor of the
transformer, the lower the value of the shunt resistor and
vice versa.
It should be noted that for high resistive values, the signal
distortion in the current transformer is low, but the thermal
noise in the resistor is high. On the other hand, for low
resistive values, the thermal noise is lower, but the power
dissipation of the resistor is higher. In addition, the current
transformer may distort the signal if the load on the second-
ary winding is too high.
As a compromise, the example wiring uses current trans-
formers with a conversion factor of 2500 and shunt
resistors of 22 ohms. The full-scale current rating is 80A, at
which the RMS voltage at the ADC input is:
The peak amplitude at the ADC input is then:
It should be noted that the current measured by the
chipset is the main current. In a Start Connection, this is of
no importance, since the phase and main currents
are equal, but in the Delta-configuration this must be
accounted for.
Measurement Method
The DSP uses the same calculation methods, regardless of
type of connection, since there is no special mode setting
for connecting the device to a Delta load. The differences in
measurement method, as compared to the default Star
connection are due to the wiring of the front end and the
conditioning of signal amplitudes.
Using the front end connections illustrated in the schematic
on page 7, the DSP will measure phase voltages and main
currents for each phase. The measurement results, avail-
able via the data registers, do not necessarily give
straightforward indications on the condition of the phase
loads in a Delta connection. For example, the RMS values
of phase voltages and main currents cannot be used to
derive the power consumption of one phase load, since the
Delta-wired load is connected between two phase nodes,
as shown in Figure 3.
The DSP will give measurement results as if the load was
connected in Star. Measurement results must therefore be
related to the equivalent Star connection as shown in
equivalent connection are not the same as those for the
actual Delta connection.
For example, the active power readings for phase one indi-
cates how much power has been consumed in the
equivalent load of Z1 and not how much power has been
consumed in any of the true loads of the Delta connection.
0.704V
2500
80A
22
M
I
R
M
I
R
M
I
RU 3
9
2
8
1
7IN =×=×=×=×=
0.9956VU2u
ˆININ =×=
VIRTUAL GROUND POTENTIAL
P1
U
P2
U
P3
U
1
U
2
U
3
U
1
I
2
I
3
I
P1
I
P3
I
P2
I
31
Z12
Z
23
Z
Meter Front End Design
5
Figure 4. Equivalent, Unbalanced Star Connection without Neutral Wire
Summary
In Delta-configuration, data registers should be interpreted as shown in Table 2.
VIRTUAL GROUND POTENTIAL
P1
U
P2
U
P3
U
1
U
2
U
3
U
1
IP1
I
1
Z
2
Z
3
Z
=
2
IP2
I
=
3
IP3
I
=
Table 2. Delta-configuration
Variable Register Measure Note
P1
P2
P3
0
1
2
Active Power Power consumed in equivalent phase load
Q1
Q2
Q3
3
4
5
Reactive Power Power consumed in equivalent phase load
S1
S2
S3
6
7
8
Apparent Power Power consumed in equivalent phase load
PF1
PF2
PF3
9
10
11
Power Factor Between main current and phase voltage
WPE 12 Active Power Exported
WPI 13 Active Power Imported
WQI 14 Reactive Power, Ind. Load
WQC 15 Reactive Power, Cap. Load
TIME 16 Time Since Reset Not affected
FREQ 17 Line Frequency Not affected
U1
U2
U3
19
20
21
RMS Voltage Phase voltage
I1
I2
I3
22
23
24
RMS Current Main current
Meter Front End Design
6
Typically, it is of no importance if the individual phase pow-
ers reported are relative to a Star or Delta connection, as
long as the overall power readings are correct. Regardless
if the chipset is wired to a default Star load or the Delta
load, as illustrated in the schematic on page 7, the overall
power readings are always obtained by summing the indi-
vidual phase registers.
When wired to a Delta load, it must be noted that individual
phase power information is correct only in the special case
when both the generator and the load are balanced. If the
load or supply is unbalanced (even if the generator is bal-
anced, impedances in the transmission lines may result in
unbalanced voltages), then only the sum of the individual
phase power readings will be valid for the Delta connection.
Individual phase powers cannot be evaluated for an unbal-
Meter Front End Design
7
Figure 5. Front End Connection Schematic
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